Wall gas furnace

Hi, if you have high voltage going to the unit and it will not fire up, then you are not getting control voltage to it. It takes 2 power sources to run it, high voltage for all of the motors and compressor, but 24 volts from your thermostat to fire it up.I dont know if you have a gas furnace, heat-pump or what. If its a gas split, you will have only 2 small thermostat wires coming into the contactor coil to fire it and close it to send power to all else.You may have a bad stat, broken stat wire, or a faulty transformer, but you have lost your 24 volts somewhere in the unit, and the 1st place to check would be the stat. Please keep me posted.
Shastalaker7 A/C, & Heating Contractor

Hi, if you have no power to the stat, go to the furnace if this is a split system. Depending on the year you have, this unit may have the main circuit board with a 3 to 5 amp control fuse plugged into it. If this is what you have, check this fuse, it is the same as the fuses in your car. If you don't have this circuit board, locate the transformer where your thermostat wires come in and check for power at both the 120 volt primary side ( power in ) and see if you have 24 volts coming out of the secondary side of it. If no power in, check the circuit breaker to the furnace. If there is power in and nothing out, the transformer is gone. Check close for any breaks in the stat wires at the furnace also. I may need to have the model and serial numbers, and if its a split gas fired or a heat-pump unit if this information does not apply to your unit. Let me know.
Shastalaker7
A/C, & Heating Contractor.

The first thing i would do is to make sure the t-stat is good.One easy way to do this is to remove stat cover then remove the screws that hold it to the wall. Disconnect t-stat wires making sure you note location. There should be a red and white wire for heat so put these two together and if your furnace comes on then the stat needs replaced. If it does not you need to check further to make sure you have power to your furnace.If the furnace does come on with these two wires together the furnace will run non stop. Hope this helps you. Thank you

Good Morning, and yes you said it. Anytime a stat doesn't come on when it is set for a call for heat or cooling, the sensor on the subbase of the stat has lost its ability to sense the correct temperature. I have seen mechanical stats last as long as 25 plus years, but that's truly amazing for a digital stat to last that long. Yes, replace the thermostat. I don't know what kind of system you have, but if its a gas furnace with an a/c, the wiring will look like this. You will have terminals marked R, Red, for the 24 volt hot wire, G, Green, Y, Yellow, and W, White for heat. Going back to the red wire, it should have RC and RH on it. There will be a normally red wire connecting the 2 terminals.The Red wire coming in can connect to either one, as they are all together. The new stat will come with the new Red jumper wire attached. So, if you do have a gas heater, this is very easy to install. The only terminals you will need to use are RH to RC for the incoming red, Y, G, and W for heat. Y is for cool, and G is for fan. Very easy. Now if you have a heat-pump, they have many more terminals and can be a problem wiring them. If you have gas, forget the heat pump. I wont confuse you by going there. If it is a heat-pump, contact me in the comment box to let me know, and I can help you there also. Get a new stat at Home Depot, Lowe's Hardware, or O.S. Hardware. You want a 5 day, 2 day programmable Heating and cooling thermostat, 24 volts.It will have everything you need. See if it takes batteries for the display, they may not come with those, it will say on the box. Turn off your power to heater, so you won't short out the new one.They come in 2 sections, first the stat that just snaps on, and the subbase that is screwed to the wall. It will have the wiring terminals on it. Remove a couple of screws and loosen wires and remove. Be sure you write down how the wires go in case they used different colors, you never know. You will be all set. Store the manual in a safe place for future use. I wish you the best.
Shastalaker7
A/C, Heating, & Refrigeration Contractor.
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If the unit fires but the main blower fan doesn't come on the the unit is shutting off on a temp high limit switch and it sounds like the problem is the main blower fan motor or control

Did this problem just start? Or has it been a long term issue? Check for broken ductwork, return air pulling outside air in. Check supply air temps, should be 100-140° or more. And what is the outdoor temp? If it's 0° outside, 74° is doing good. Is the home insulated properly? And there is a small hole behind the t-stat where the control wire comes through the wall. Plug that hole, seal that hole to prevent cold air from fooling the t-stat. The furnace has 3 fan speeds and it may be in low speed. If so it should be in medium speed. I'd suggest a service tech doing this for you unless you are experienced in furnace repair.

Hi, What is happening is the control wiring from this thermostat is touching on the heat side from somewhere in the wall back to the furnace, so it calls for heat. It gets its power from the transformer on the furnace. Try disconnecting it at the furnace and it will shut down. If this is heat only, should have only 2 conductors and they would have to be touching somewhere. Look carefully at the wiring behind the stat and around the furnace for it may have melted together or bare spots where it is touching. This is the only thing that would make it come on with out the stat. Please let me know what you find. You can also ohm the red and white out to see if there is continuity, and if so, they are touching and you will either need to find it or run a new set of stat wires to solve this. Get back to me. Good Luck,
Shastalaker7

observe the sequence of operation. stat calls for heat and sends a signal, to furnace. check for 24 volts to common and white wire at furnace.first the forced draft motor starts and should have 120 volts to the plug it attaches to. after forced draft motor starts the pressure or vacuum switch closes should show no 24 volt when checked across terminals. shortly after the ignitor glows and shortly after click the main gas valve opens and main burners fire. shortly after the indoor fan comes on and hot air blows into the ductwork. follow the chain of events to see what doesnt happen and it is a clue to whats broke. maybe start by switching fan switch on stat to on. if fan comes on you have high and low voltage thereby eliminating the stat power and transformer as a problem. if you can see the ignitor look for it to be cracked . look close dont touch any part of it .it is powered by 120 volts when energized from board. dont forget when you take the door off there is no power to the furnace so be prepared to tape the safety switch closed when you are diagnosing.

you should have 24 volts between yellow and white wires at the stat when the stat is set to heat and set as hot as it will go. burners should light first then 3 to 5 minutes later the fan motor should fire up. if burner doesn't light check the wires going into the top of the gas valve see if you have 24 volts arriving there from the stat. if you do then the gasvalve is probably bad. you can bypass the action of the stat by jumping red to white on 90 percent of the stats made and in a few seconds the gas valve should click open gas flow to main burner where pilot ignites it. another easy check you can do is flip stat fan switch to on. fan should come on at once. if not then you have either no high voltage to unit or no low voltage ( as in a bad transformer). if the fan comes on you know you have high volts and low volts...by cover i assume you mean the decorative cover but there second cover with the mercury bulbs and all is still there. if you have only the bare subbase then new stat time. sounds like an old furnace if it has a standing pilot, it should be checked for cracked heat exchanger by a pro with a n electronic carbon monoxide detector. NOT a diy item due to the real danger of co poisoning.

You must check for voltage at the stat. If voltage present with no display the stat is bad. No voltage, you gotta find out where it stops.

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